1. Field of the Invention
The invention relates to digital television system, and in particular, to decision feedback equalizers with adjustable equalizer span.
2. Description of the Related Art
FIG. 1 shows a conventional decision feedback equalizer. Received sample x(n) denotes a sampled symbol corresponding to time n. The linear equalizer (LE) 102 filters the received sample x(n) to generate part of the equalizer output y(n). A slicer 106 slices the equalizer output y(n) to generate a sliced sample d(n), and the decision feedback equalizer (DFE) 104 filters the sliced sample d(n) to contribute the equalizer output y(n) by an adder 108. The linear equalizer 102 and decision feedback equalizer 104 each comprises a delay line formed by a plurality of taps, and the received sample x(n) and sliced sample d(n) shifting in the delay lines are multiplied with coefficients to generate the equalizer output. The coefficients are also recursively updated by another algorithm such as the least-mean-square (LMS) algorithm. This architecture is typical and widely known, and detailed implementation varies with application.
FIG. 2 shows an equalizer span formed by linear equalizer taps and decision feedback equalizer taps. Each tap in the LE delay line 202 stores a sample value. The tap storing received sample x(n) is referred to as the main tap. Number of taps anterior to the main tap is LEL, and number of taps posterior to the main tap is LER. An equalizer output y(n) is the combination of the LE delay line 202 output and the DFE delay line 204 output, expressed as:
                              y          ⁡                      (            n            )                          =                                            ∑                              k                =                                  -                  LEL                                            LER                        ⁢                                          f                k                            ⁢                              x                ⁡                                  (                                      n                    -                    k                                    )                                                              +                                    ∑                              k                =                1                            DFE                        ⁢                                          b                k                            ⁢                              d                ⁡                                  (                                      n                    -                    k                                    )                                                                                        (        1        )            
Where LEL is the number of taps anterior to the main tap, and DFE is the number of DFE delay line 204 taps. The LEL and DFE are predetermined numbers, and the equalizer span is equal to LEL+DFE+1. fk and bk are coefficients of the linear equalizer 102 and decision feedback equalizer 104, respectively.
Since the channel conditions are unpredictable, a predetermined main tap allocation scheme, which corresponds to a fixed equalizer span, may not always render good SNR results. Dynamic main tap allocation methods have been proposed. However, some methods require significant computation resources, making the hardware implementations unacceptable. Therefore a more efficient improvement is desirable.